LT3752-1_15 Datasheet, PDF (39/52 Pages) Linear Technology – Active Clamp Synchronous Forward Controllers with Internal Housekeeping Controller

LT3752/LT3752-1
APPLICATIONS INFORMATION
The selection of the main transformer will depend on the
applications requirements : isolation voltage, power level,
maximum volt-seconds, turns ratio, component size, power
losses and switching frequency.
Transformer construction using the planar winding technol-
ogy is typically chosen for minimizing leakage inductance
and reducing component height. Transformer core type is
usually a ferrite material for high frequency applications.
Find a family of transformers that meet both the isolation
and power level requirements of the application. The next
step is to find a transformer within that family which is
suitable for the application. The subsequent thought pro-
cess for the transformer design will include :
(1) Secondary turns (N_S), core losses, temperature
rise, flux density, switching frequency
(2) Primary turns (N_P), maximum duty cycle and reset
voltages
(3) Copper losses
The expression for secondary turns (N_S) is given by,
N_S = 10⁸ V_OUT/(f_OSC • A_C • B_M)
where,
A_C = cross-sectional area of the core in cm²
B_M = maximum AC flux density desired
For flux density, choose a level which achieves an accept-
able level of core loss/temperature rise at a given switching
frequency. The transformer data sheet will provide curves
of core loss versus flux density at various switching fre-
quencies. The data sheet will also provide temperature rise
versus core loss. While choosing a value for BM to avoid
excessive core losses will usually allow enough headroom
for flux swing during input / load transients, still make
sure to stay well below the saturation flux density of the
transformer core. If needed, increasing N_S will reduce flux
density. After calculating N_S, the number of primary turns
(N_P) can be calculated from,
N_P = N_S • D_MAX V_IN(MIN)/V_OUT
where,
V_IN(MIN) = minimum system input voltage
D_MAX = maximum switch duty cycle at V_IN(MIN) (typically
chosen between 0.6 and 0.7)
At minimum input voltage the converter will run at a maxi-
mum duty cycle D_MAX. A higher transformer turns ratio
(N_P/N_S) will create a higher D_MAX but it will also require
higher voltages at the drain of the primary side switch to
reset the transformer (see previous sections Lo side Active
Clamp Topology and Hi side Active Clamp Topology). D_MAX
values are typically chosen between 0.6 and 0.7. Even for
a given D_MAX value, the loop must also provide protection
against duty cycles that may excessively exceed D_MAX
during transients or faults. While most converters only
provide a fixed duty cycle clamp, the LT3752/LT3752-1
provide a programmable maximum duty cycle clamp D_VSEC
that also moves inversely with input voltage.
The resulting function is that of a programmable volt-
second clamp. This allows the user to choose a transformer
turns ratio for D_MAX and then customize a maximum duty
cycle clamp D_VSEC above D_MAX for safety. D_VSEC then
follows the natural duty cycle of the converter as a safety
guardrail (see previous section Programming Duty Cycle
Clamp).
After deciding on the particular transformer and turns ratio,
the copper losses can then be approximated by,
P_CU = D • I(Load)_(MAX)2 (R_SEC + (N_S/N_P)² R_PRI)
where,
D = switch duty cycle (choose nominal 0.5)
I(Load)_(MAX) = maximum load current
For more information www.linear.com/LT3752
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